The invention relates to a process for material treatment by means of laser in a liquid-filled space, in which a material removal occurs by pulsewise laser emission, as well as to an apparatus for the execution of this process.
To the state of the art there belongs the practice, by means of a laser to remove material at an intended place of application by pulsewise laser emission. A special use of such a material treatment by means of laser is material removal under exclusion of of liquid. This application makes possible a finely sensitive and precise material removal such as is not feasible in this manner with other which, for example chipping processes or spark-erosive processes--which presumes the electric conductivity of the material to be removed. Thus with the known material treatment process by means of laser vessels or line systems can be treated, for example in the event of cloggings or deposits, without it being necessary to remove the liquid ordinarily present in them.
Also with this material processing, however, problems arise. Thus, for example, by ablation or by disruption in the environment of the place of application very high pressures can arise. These pressures arise through thermal expansion of the material to be removed and/or of the surrounding fluid when material is converted from the solid or liquid state into the gaseous state. In consequence of these very high pressures occurring briefly and locally, a pressure wave is radiated into the surrounding fluid and a cavitation bubble is generated, through the dynamics of which there can in individual cases occur damages to the surrounding material.
Because of the mass inertia of the surrounding liquid, a conduction system that is open per se can act in this case as a closed system. If a cavitation bubble is generated by rapid evaporation, then the mass inertia of the surrounding liquid must be overcome. Thereby there arise high pressures in the bubble interior, which lead to an acceleration outward of the bubble walls. When the interior pressure of the bubble has fallen to environmental pressure, the kinetic energy of the outerward-flowing liquid is maximum. Because of the inertia forces the bubble sweeps beyond the equilibrium point. Pressure and density in the interior of the bubble fall to very low values before the bubble collapses under the outside pressure. Damages in the bubble environment can arise through the excessively high pressure at the beginning of the bubble expansion, through the kinetic energy of the out-ward flowing liquid and through the collapse of the bubble.
Through the local pressure increase and the bubble dynamics caused thereby there can occur an overstraining of the components or line walls lying in this region. Further, through the aforementioned pressure increases there is present the danger that in the surrounding liquid gas will be dissolved which in the subsequent pressure drop will be liberated again uncontrolled.
Underlying the invention is the problem of further developing a process according to the category in such a way that these partial strong pressure rises are reduced and also the cavitation phenomena and the aforementioned harmful effects are avoided, at least, however, clearly reduced. Further, there are to be readied suitable devices for the execution of the process of the invention.
The process part of the above problem is solved according to the invention by the means that immediately before the laser treatment gas is fed in or generated to displace the liquid present there. Underlying the present invention is the thought that the reduction of the pressure increase can occur effectively by the means that the liquid in the immediate environment of the place of application is expelled by gas, which is either supplied to this place from outside or else is generated in a suitable manner in this place immediately before the laser treatment. And, namely, the reduction of the pressure increase is all the greater the more liquid is removed before the application from the volume to be regarded in this case as closed-off. The pressure increase resulting from the generation of the gas or the supplying of the gas and the corresponding flow velocities can be kept relatively low with suitable execution.
While in the process according to the state of the art described in the preface it is not always an inconsiderable part of the liquid present around the application place which is evaporated, since the evaporation temperature of the surrounding liquid is as a rule substantially lower than that of the material to be removed, the otherwise usual strong pressure increase is responsible for not inconsiderable parts of this evaporation of the liquid, which with the process of the invention can be substantially avoided or at least strongly reduced. With the process of the invention neither is energy absorbed in the laser irradiation of the application place nor is energy lost by heat conduction from the place of application into the surrounding liquid. Furthermore, through the acoustic impedance leap on the surface of the target material it is made possible for spallation effects to arise in the target material. The bubble generation before the target material makes possible, therefore, besides the pressure reduction, also an an improvement in the ablation efficiency.